1. Field of the Invention
The invention relates to a pneumatic spring percussion mechanism in accordance with the preamble of patent claim 1 and to a percussion hammer and/or hammer drill in which a pneumatic spring percussion mechanism of this type is used.
2. Description of the Related Art
Pneumatic spring percussion mechanisms, in particular for use in percussion hammers and/or hammer drills, are generally known. In a pneumatic spring percussion mechanism of this type a driving piston is set into an oscillating axial movement by a suitable drive, for example a crank assembly connected to an electric motor. In a particularly advantageous design, the driving piston is hollow. A percussion piston is inserted into the cavity in the driving piston with a pneumatic spring forming at least between an end surface of the percussion piston and the cavity in the driving piston. Owing to the inertia of the percussion piston, the movement of the driving piston initially produces a positive pressure in the pneumatic spring, which pressure is used to drive the percussion piston in the direction of a tool fastened on the hammer/drill. After the percussion piston has struck against the tool or against a riveting set arranged between the percussion piston and tool, it rebounds. In the pneumatic spring the reverse movement of the driving piston, which movement is produced by the crank assembly, causes suction to be exerted on the percussion piston, the suction assisting the recoil produced during the blow and exerted on the percussion piston whereupon the percussion piston moves away from the tool. After the upper dead center of the driving piston is reached, said driving piston again moves in the opposite direction toward the tool, retards the percussion piston, which is still in the reverse movement, and accelerates it again onto the tool in order to carry out the next blow.
Apart from the single-sided pneumatic spring percussion mechanisms which have just been described, percussion mechanisms having a double pneumatic spring are also known in which a pneumatic spring is formed between the percussion piston and driving piston not just behind the percussion piston--as seen in the percussion direction but also in front of the percussion piston. A two-sided pneumatic spring percussion mechanism of this type permits reliable starting and idling behavior.
FIG. 5 shows an example of a conventional percussion mechanism having a double pneumatic spring. A percussion piston 2 is inserted in a driving piston 1 which can be moved axially to and fro, for example, by a crank assembly (not shown) in a percussion mechanism housing (not shown). A front pneumatic spring 3 is formed in front of the percussion piston 2 and a rear pneumatic spring 4 is formed behind the percussion piston 2. The front and the rear pneumatic springs 3, 4 are supplied with air via a ventilation slot 5, which leads to the surroundings or into the crank housing, in the driving piston 1.
Owing to its inertia the percussion piston 2 follows the oscillation movement of the driving piston 1 with a constant time delay. As a result, during the forward movement of the driving piston 1 (direction of movement to the left in FIG. 5) an air pressure builds up in the rear pneumatic spring 4, which pressure finally likewise drives the percussion piston 2 in a forward direction where the percussion piston 2 strikes against a tool (not shown) or against a riveting set (not shown) and carries out the blow. Subsequently, because of the action of the crank assembly the driving piston 1 passes into a reverse movement (to the right in the figure), as a result of which a positive pressure builds up in the front pneumatic spring 3, which pressure likewise drives the percussion piston 2 in the reverse movement and reinforces the recoil produced by the blow. During each stroke movement of the percussion piston 2 a connection is produced between the ventilation slot 5 and one of the respectively unstressed pneumatic springs 3, 4, which makes it possible to even out the air. However, at low ambient air pressure, for example, at great operating heights, it is possible that the amount of air penetrating into the particular pneumatic spring 3, 4 is not sufficient to then build up a sufficiently strong air cushion.
Pneumatic spring percussion mechanisms therefore generally have the problem of the functioning of the percussion mechanism depending on the amount of air which is available, i.e. on the ambient air pressure or on the ambient air density. It is thus possible, for example, to optimally configure a percussion mechanism for use at sea level while the same percussion mechanism can only be used to a restricted extent at great heights.
This is because there is less amount of air available at relatively great heights to fill the pneumatic spring. The strain on the pneumatic spring and on the other elements of the percussion mechanism drive is thereby increased which in extreme cases can result in the percussion mechanism being damaged by the percussion piston and the driving piston knocking together, if the pneumatic spring lying between them contains insufficient air to ensure an adequate pressure for separating the percussion piston and driving piston.
German Patent 255 977 discloses a pneumatic spring percussion mechanism of the generic type which essentially has the features of the pneumatic spring percussion mechanism described with reference to FIG. 5.
CH 567 911 describes a percussion mechanism having a single-sided pneumatic spring, in which a percussion piston and a driving piston are arranged in an axially moveable manner in a fixed cylinder. Between the driving piston and the percussion piston there is formed a cavity which serves as the pneumatic spring and after each blow is supplied with air via a groove which is formed on the inside of the cylinder and extends in the percussion direction. The air is supplied from the surroundings via gaps in the housing to a cylinder interior which, as seen in the percussion direction, is arranged in front of the percussion piston and when the percussion piston is positioned appropriately can communicate with the pneumatic spring cavity through the groove.
The problem of an insufficient supply of air to the pneumatic spring if the percussion mechanism is used, for example, at great heights, also occurs in the case of the two last-mentioned pneumatic spring percussion mechanisms.